Catalyst layer

ABSTRACT

A catalyst layer including: (i) a platinum-containing electrocatalyst; (ii) an oxygen evolution reaction electrocatalyst; (iii) one or more carbonaceous materials selected from the group consisting of graphite, nanofibres, nanotubes, nanographene platelets and low surface area, heat-treated carbon blacks wherein the one or more carbonaceous materials do not support the platinum-containing electrocatalyst; and (iv) a proton-conducting polymer and its use in an electrochemical device are disclosed.

FIELD OF THE INVENTION

The present invention relates to a catalyst layer, which shows toleranceto high voltage situations, such as cell reversal and start-up shut-downincidences that occur in fuel cells. The invention also relates toelectrodes, catalyst-coated membranes and membrane electrode assembliescomprising the catalyst layer.

BACKGROUND OF THE INVENTION

A fuel cell is an electrochemical cell comprising two electrodesseparated by an electrolyte. A fuel, e.g. hydrogen, an alcohol such asmethanol or ethanol, or formic acid, is supplied to the anode and anoxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemicalreactions occur at the electrodes, and the chemical energy of the fueland the oxidant is converted to electrical energy and heat.Electrocatalysts are used to promote the electrochemical oxidation ofthe fuel at the anode and the electrochemical reduction of oxygen at thecathode.

Fuel cells are usually classified according to the nature of theelectrolyte employed. Often the electrolyte is a solid polymericmembrane, in which the membrane is electronically insulating butionically conducting. In the proton exchange membrane fuel cell (PEMFC)the membrane is proton conducting, and protons, produced at the anode,are transported across the membrane to the cathode, where they combinewith oxygen to form water.

A principal component of the PEMFC is the membrane electrode assembly(MEA), which is essentially composed of five layers. The central layeris the polymer ion-conducting membrane. On either side of theion-conducting membrane there is an electrocatalyst layer, containing anelectrocatalyst designed for the specific electrolytic reaction.Finally, adjacent to each electrocatalyst layer there is a gas diffusionlayer. The gas diffusion layer must allow the reactants to reach theelectrocatalyst layer and must conduct the electric current that isgenerated by the electrochemical reactions. Therefore the gas diffusionlayer must be porous and electrically conducting.

Electrocatalysts for fuel oxidation and oxygen reduction are typicallybased on platinum or platinum alloyed with one or more other metals. Theplatinum or platinum alloy catalyst can be in the form of unsupportednanoparticles (such as metal blacks or other unsupported particulatemetal powders) or can be deposited as even higher surface area particlesonto a conductive carbon substrate or other conductive material (asupported catalyst).

Conventionally, the MEA can be constructed by a number of methodsoutlined hereinafter:

-   -   (i) The electrocatalyst layer may be applied to the gas        diffusion layer to form a gas diffusion electrode. A gas        diffusion electrode is placed on each side of an ion-conducting        membrane and laminated together to form the five-layer MEA;    -   (ii) The electrocatalyst layer may be applied to both faces of        the ion-conducting membrane to form a catalyst coated        ion-conducting membrane. Subsequently, a gas diffusion layer is        applied to each face of the catalyst coated ion-conducting        membrane.    -   (iii) An MEA can be formed from an ion-conducting membrane        coated on one side with an electrocatalyst layer, a gas        diffusion layer adjacent to that electrocatalyst layer, and a        gas diffusion electrode on the other side of the ion-conducting        membrane.

Typically tens or hundreds of MEAs are required to provide enough powerfor most applications, so multiple MEAs are assembled to make up a fuelcell stack. Field flow plates are used to separate the MEAs. The platesperform several functions: supplying the reactants to the MEAs; removingproducts; providing electrical connections; and providing physicalsupport.

High electrochemical potentials can occur in a number of real-lifeoperational situations and in certain circumstances can cause damage tothe catalyst layer/electrode structure, primarily due to corrosion ofany carbon present in the layer, such as the support material for thecatalyst. Such situations are well documented but include:

-   -   (i) Cell reversal: fuel cells occasionally are subjected to a        voltage reversal (cell is forced to the opposite polarity). In        addition to the loss of power associated with one or more cells        going into voltage reversal, undesirable electrochemical        reactions may occur which detrimentally affect fuel cell        components.    -   (ii) Start-up shut-down: for many fuel cells it is not practical        or economic to provide purging of hydrogen from the anode gas        space with an inert gas such as nitrogen during shut down. This        means there may arise a mixed composition of hydrogen and air on        the anode whilst air is present on the cathode. Similarly, when        a cell is re-started after being idle for some time, air may        have displaced hydrogen from the anode and as hydrogen is        re-introduced to the anode, again a mixed air/hydrogen        composition will exist whilst air is present at the cathode.        Under these circumstances an internal cell can exist, which        leads to high potentials on the cathode.

Solutions proposed to address the problems associated with incidences ofhigh electrochemical potentials include employing a catalyst that ismore resistant to oxidative corrosion than conventional catalysts andincorporating an additional catalyst composition for electrolysing water(water electrolysis thus occurs in preference to corrosion of any carbonsupport).

SUMMARY OF THE INVENTION

For fuel cells to become commercially viable, particularly forautomotive use, it is necessary to provide a high performance and stablecatalyst layer, but with a low platinum loading. Using an unsupported“Pt black” catalyst in the catalyst layer compared to a carbon supportedplatinum catalyst can provide a more durable catalyst layer by avoidingthe problems associated with carbon corrosion that occurs as a result ofthe high potentials caused by incidences of cell reversal and start-upshut-down. However, the present inventors have found that to produce aPt black catalyst layer with the required structure for satisfactoryoperation and with sufficient uniformity and continuity of coating ofthe catalyst layer on the substrate, the platinum loading required ismuch higher than the requirements for commercialisation of many fuelcell applications, and in particular for automotive applications. Thepresent inventors have surprisingly found that incorporation of a highlyelectrically conductive and corrosion resistant carbonaceous materialinto the catalyst layer enables suitable catalyst layer structures to beproduced, that are also of excellent uniformity and continuity and thathave a platinum loading in the catalyst layer consistent with automotiverequirements.

The present invention provides a catalyst layer. The catalyst layercomprises: (i) a platinum-containing electrocatalyst; (ii) an oxygenevolution reaction electrocatalyst; (iii) one or more carbonaceousmaterials selected from the group consisting of graphite, nanofibres,nanotubes, nanographene platelets and low surface area, heat-treatedcarbon blacks, wherein the one or more carbonaceous materials do notsupport the platinum-containing electrocatalyst; and (iv)proton-conducting polymer.

The invention also provides an electrode, either an anode or cathode,comprising a gas diffusion layer and the catalyst layer of theinvention.

The invention also provides a catalyst-coated membrane comprising aproton-conductive membrane and the catalyst layer of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the Beginning of Life (BOL) performance of Example 1 andComparative Example 1.

FIG. 2 shows the performance of Example 1 and Comparative Example 1after incidences of cell reversal.

DETAILED DESCRIPTION OF THE INVENTION

Preferred and/or optional features of the invention will now be set out.Any aspect of the invention may be combined with any other aspect of theinvention, unless the context demands otherwise. Any of the preferred oroptional features of any aspect may be combined, singly or incombination, with any aspect of the invention, unless the contextdemands otherwise.

Electrocatalyst

The electrocatalyst comprises platinum.

The platinum may be alloyed or mixed with one or more other platinumgroup metals (ruthenium, rhodium, palladium, osmium or iridium), gold,silver or a base metal or an oxide of one or more other platinum groupmetals, gold, silver or a base metal.

The platinum may be alloyed or mixed with one or more of ruthenium,nickel, cobalt, chromium, iridium, copper, iron, zinc, osmium, niobium,tantalum, vanadium, tin, titanium and rhodium.

The electrocatalyst is unsupported i.e. it is present as the ‘black’.

Alternatively, the electrocatalyst is supported on a non-carbonaceoussupport (such as titania, niobia, tantala, tungsten carbide, hafniumoxide or tungsten oxide).

Alternatively, the electrocatalyst is supported on the oxygen evolutionreaction catalyst.

In one embodiment, the platinum-containing electrocatalyst isunsupported, supported on a non-carbonaceous support or supported on theoxygen evolution reaction catalyst.

The electrocatalyst may be made by methods known to those in the art,for example by wet chemical methods.

In one embodiment, the electrocatalyst is a hydrogen oxidation reactioncatalyst. The hydrogen oxidation reaction catalyst is platinum which maybe alloyed with one or more other metals, such as osmium, ruthenium,niobium, tantalum, vanadium, iridium, tin, titanium or rhodium.

In a second embodiment, the electrocatalyst is an oxygen reductionreaction catalyst. The oxygen reduction reaction catalyst is platinumwhich may be alloyed with one or more other metals, such as nickel,cobalt, chromium, palladium, iridium, copper, iron or zinc.

Oxygen Evolution Reaction Electrocatalyst

The oxygen evolution reaction (OER) catalyst is any catalyst known tothose skilled in the art which catalyses the oxygen evolution reaction.

The oxygen evolution reaction catalyst suitably does not compriseplatinum.

The oxygen evolution reaction catalyst may comprise ruthenium orruthenium oxide or iridium or iridium oxide or mixtures thereof.

The oxygen evolution reaction catalyst may comprise iridium or iridiumoxide and one or more metals M or an oxide thereof. M is a transitionmetal (other than iridium or ruthenium) or tin.

M may be a Group 4 metal: titanium, zirconium or hafnium.

M may be a Group 5 metal: vanadium, niobium or tantalum

M may be a Group 6 metal: chromium, molybdenum or tungsten.

M may be tin.

M may be selected from the group consisting of tantalum, titanium,zirconium, hafnium, niobium and tin; preferably tantalum, titanium andtin.

The iridium or oxide thereof and the one or more metals (M) or oxidethereof may either exist as mixed metals or oxides or as partly orwholly alloyed materials or as a combination thereof. The extent of anyalloying can be shown by x-ray diffraction (XRD).

The atomic ratio of iridium to (total) metal M in the oxygen evolutioncatalyst is from 20:80 to 99:1, suitably 30:70 to 99:1 and preferably60:40 to 99:1.

Such oxygen evolution catalysts may be made by methods known to those inthe art, for example by wet chemical methods.

The oxygen evolution reaction catalyst may comprise a mixed metal oxideof formula

(AA′)_(a)(BB′)_(b)O_(c).

-   -   wherein A and A′ are the same or different and are selected from        the group consisting of yttrium, lanthanum, cerium,        praseodymium, neodymium, promethium, samarium, europium,        gadolinium, terbium, dysprosium, holmium, erbium, thulium,        ytterbium, lutetium, magnesium, calcium, strontium, barium,        sodium, potassium, indium, thallium, tin, lead, antimony and        bismuth; B is selected from the group consisting of Ru, Ir, Os,        and Rh; B′ is selected from the group consisting of Ru, Ir, Os,        Rh, Ca, Mg or RE (wherein RE is a rare earth metal); c is from        3-11; the atomic ratio of (a+b):c is from 1:1 to 1:2; the atomic        ratio of a:b is from 1:1.5 to 1.5:1.

A and A′ may be selected from the group consisting of: sodium,potassium, calcium, strontium, barium, lead and cerium.

B may selected from the group consisting of Ru, Ir, Os, Rh (suitably Ruand Ir) having an oxidation state of from 3⁺ to 6⁺, includingintermediate partial oxidation states.

B′ may be selected from the group consisting of Ru, Ir, Os, Rh (suitablyRu and Ir) having an oxidation state of from 3⁺ to 6⁺, includingintermediate partial oxidation states, Ca, Mg, RE (wherein RE is ashereinafter defined), indium, thallium, tin, lead, antimony and bismuth.

c is from 3-11. Since the atomic ratio of (a+b):c is known, the value of(a+b) can be determined. Similarly, since the atomic ratio of a:b andthe value of (a+b) is known, the values of a and b can be determined.

Specific examples of crystalline metal oxides which may be used as theoxygen evolution catalyst include, but are not limited to: RERuO₃;SrRuO₃; PbRuO₃; REIrO₃; CaIrO₃; BaIrO₃; PbIrO₃; 5rIrO₃; KIrO₃;SrM_(0.5)Ir_(0.5)O₃; Ba₃LiIr₂O₉; Sm₂NaIrO₆; La_(1.2)Sr_(2.7)IrO_(7.33);Sr₃Ir₂O₇; Sr₂Ir₃O₉; SrIr₂O₆; Ba₂Ir₃O₉; BaIr₂O₆; La₃Ir₃O₁₁; RE₂Ru₂O₇;RE₂Ir₂O₇; Bi₂Ir₂O₇; Pb₂Ir₂O₇; Ca₂Ir₂O₇; (NaCa)₂Ir₂O₆; (NaSr)₃Ir₃O₁₁;(NaCe)₂Ir₂O₇; (NaCe)₂Ru₂O₇; (NaCe)₂(RuIr)₂O₇.

In the above specific examples: RE is one or more rare earth metalsselected from the group consisting of: yttrium, lanthanum, cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium; M isCa, Mg or RE (where RE is as defined before).

These crystalline mixed metal oxides may be prepared by methods known inthe art, such as described in WO2012/080726.

The oxygen evolution catalyst may be unsupported.

Alternatively, the oxygen evolution catalyst may be supported on atleast some of the one or more carbonaceous materials selected from thegroup consisting of graphite, nanofibres, nanotubes, nanographeneplatelets and low surface area, heat-treated carbon blacks, or on anon-carbonaceous support (such as titania, niobia, tantala, tungstencarbide, hafnium oxide or tungsten oxide).

Carbonaceous Material

The electrically conductive material may be graphite. Graphite is acrystalline allotropic form of carbon which has a layered, planarstructure; in each layer, the carbon atoms are arranged in a hexagonalcrystalline lattice. Graphite exists either in natural or synthetic formand is suitably used in its synthetic form.

The graphite may be particulate graphite, e.g. flake graphite, sphericalgraphite or graphite powder. The particle size of as-received graphitematerials can vary over a wide range for different grades, typicallyfrom as low as 3 micron to as high as 500 micron diameter. The preferredgraphite materials have a D50 (i.e. 50% of the particles are below aspecific particle size) particle size range of from 3-4 micron up toaround 20 micron, but are able to be broken down into smaller particlesduring processing to form the catalyst layer structures.

The graphite has a low specific surface area (as measured using thewell-known BET method), for example a specific surface area less than 40m²/g, for example less than 20 m²/g.

Examples of such graphite materials include products available fromTimcal Graphite & Carbon under the tradenames TIMREX™ and C-NERGY™, fromBranwell Graphite Ltd such as V-SGA5 and from Alfa Aesar, such asproduct 46304.

The carbonaceous material may be fibrous or tubular nanofibres ornanotubes, such as Pyrograf III® Carbon Fiber from Pyrograf ProductsInc. or VGCF-H from Showa Denko K.K.

The carbonaceous material may be nanographene platelets, such as N008 orN006 from Angston Materials LLC.

The carbonaceous material may be a low surface area, heat-treated carbonblack, for example graphitised Vulcan XC72R, which is available fromCabot Corporation. Low surface area, heat treated carbon blacks have anamorphous structure and are obtained by heat treating a commerciallyavailable carbon black in an inert atmosphere at a temperature ofgreater than 1500° C., suitably greater than 2000° C.

The carbonaceous materials used in the present invention have a surfacearea (as measured using the well-known BET method) of less than 100m²/g, suitably less than 80 m²/g, suitably less than 40 m²/g andpreferably less than 20 m²/g.

Suitably, the one or more carbonaceous materials is selected from thegroup consisting of graphite, nanofibres, nanotubes and nanographeneplatelets; preferably, the one or more carbonaceous materials includesgraphite, and most preferably, the carbonaceous material is graphite.

The one or more carbonaceous materials do not support theplatinum-containing electrocatalyst. Suitably, the one or morecarbonaceous materials do not support the oxygen evolution catalyst.

Proton-Conducting Polymer

The proton-conducting polymer is any polymer that is capable ofconducting protons. Examples of such polymers include dispersions ofpolymers based on perfluorosulphonic acid (PFSA) polymers (such as thosesold under the trade names Nafion® (E.I. DuPont de Nemours and Co.),Aquivion® (Solvay Speciality Polymers), Aciplex® (Asahi Kasei) andFlemion® (Asahi Glass KK). Such PFSA based ion-conducting polymers areformed from the copolymerisation of tetrafluoroethylene and aperfluorinated sulphonic acid derivative.

As an alternative to PFSA ion-conducting polymers it is possible to usedispersions of ion-conducting polymers based on sulfonated orphosphonated hydrocarbon polymers, such as the polyaromatic class ofpolymers.

Catalyst Layer

The catalyst layer may comprise additional components. Such componentsinclude, but are not limited to: a hydrophobic (a polymer such as PTFEor an inorganic solid with or without surface treatment) or ahydrophilic (a polymer or an inorganic solid, such as an oxide) additiveto control water transport; an additional catalytic material for examplehaving activity for the decomposition of hydrogen peroxide (e.g. ceriaor manganese dioxide) or for improving the carbon monoxide tolerance ofthe catalyst layer (e.g. tantalum, niobium, rhodium). Any additionalcatalytic material may be supported or unsupported; if supported, it maybe supported on at least some of the one or more carbonaceous materialsselected from the group consisting of graphite, nanofibres, nanotubes,nanographene platelets and low surface area, heat-treated carbon blacks,or on a non-carbonaceous support (such as titania, niobia, tantala,tungsten carbide, hafnium oxide or tungsten oxide).

To prepare the catalyst layer the platinum-containing electrocatalyst,oxygen evolution reaction electrocatalyst, one or more carbonaceousmaterials and proton-conducting polymer, and any additional components,are dispersed in an aqueous and/or organic solvent, to prepare acatalyst ink. If required, particle break-up is carried out by methodsknow in the art, such as high shear mixing, milling, ball milling,passing through a microfluidiser etc or a combination thereof, toachieve uniformity of particle size.

After preparation of the catalyst ink, the ink is deposited onto asubstrate (e.g. gas diffusion layer, proton exchange membrane ortransfer substrate) to form the catalyst layer. The ink may be depositedby standard methods such as printing, spraying, knife over roll, powdercoating, electrophoresis etc.

The catalyst layer is suitably ≧1 μm; more suitably ≧2 μm in thickness.

The catalyst layer is suitably ≦20 μm; more suitably ≦15 μm inthickness; preferably ≦10 μm in thickness.

The loading of platinum (from the platinum-containing electrocatalyst)in the catalyst layer is ≦0.8 mg/cm², suitably ≦0.4 mg/cm².

The loading of platinum (from the platinum-containing electrocatalyst)in the catalyst layer is ≦0.01 mg/cm², suitably ≦0.025 mg/cm².

The exact loading of platinum (from the platinum-containingelectrocatalyst) in the catalyst layer is dependent upon whether thecatalyst layer is for use at the anode or the cathode and determinationof appropriate loadings will be known to those skilled in the art. Forexample, for use at the anode a platinum loading of 0.02-0.15 mg/cm² isappropriate. For use at the cathode, a higher platinum loading of0.1-0.8 mg/cm² is appropriate

Suitably, the ratio (by weight) of the oxygen evolution catalyst tototal platinum-containing electrocatalyst (platinum+any alloying metal)in the catalyst layer is from 20:1 to 1:20, preferably from 1:1 to 1:10.The actual ratio will depend on whether the catalyst layer is employedat the anode or cathode and whether the oxygen evolution catalyst isused as a support for the electrocatalyst.

The ratio (by weight) of the platinum in the platinum-containingelectrocatalyst to the one or more carbonaceous materials is from 5:95to 95:5, suitably 10:90 to 90:10 and more suitably 40:60 to 90:10.

The catalyst layer of the invention has utility in an electrochemicaldevice, and has particular utility in a membrane electrode assembly fora PEMFC.

The invention provides a gas diffusion electrode comprising a gasdiffusion layer and a catalyst layer of the invention.

To prepare a gas diffusion electrode the catalyst ink is depositeddirectly onto a gas diffusion layer (GDL) for example by one of themethods previously described. Alternatively, the catalyst layer is firstformed on a transfer substrate (such as a polymeric material such aspolytetrafluoroethylene (PTFE) polyimide, polyvinylidene difluoride(PVDF), or polypropylene (for example biaxially-oriented polypropylene,BOPP) by deposition of a catalyst ink as hereinbefore described onto thetransfer substrate. The catalyst layer may then be transferred to theGDL using a decal transfer technique known to those skilled in the art.

The GDLs are suitably based on conventional non-woven carbon fibre gasdiffusion substrates such as rigid sheet carbon fibre papers (e.g. theTGP-H series of carbon fibre papers available from Toray IndustriesInc., Japan) or roll-good carbon fibre papers (e.g. the H2315 basedseries available from Freudenberg FCCT KG, Germany; the Sigracet® seriesavailable from SGL Technologies GmbH, Germany; the AvCarb® seriesavailable from Ballard Material Products, United States of America; orthe NOS series available from CeTech Co., Ltd. Taiwan), or on wovencarbon fibre cloth substrates (e.g. the SCCG series of carbon clothsavailable from the SAATI Group, S.p.A., Italy; or the WOS seriesavailable from CeTech Co., Ltd, Taiwan). For many PEMFC applications thenon-woven carbon fibre paper, or woven carbon fibre cloth substrates aretypically modified with a hydrophobic polymer treatment and/orapplication of a microporous layer comprising particulate materialeither embedded within the substrate or coated onto the planar faces, ora combination of both, to form the gas diffusion layer. The particulatematerial is typically a mixture of carbon black and a polymer such aspolytetrafluoroethylene (PTFE). Suitably the gas diffusion layers arebetween 100 and 300 μm thick. Preferably there is a layer of particulatematerial such as carbon black combined with PTFE on the faces of the gasdiffusion layers that contact the electrocatalyst layers.

The invention also provides a catalyst coated membrane (CCM) comprisinga proton exchange membrane (having a first and second face) and acatalyst layer of the invention. The catalyst layer of the invention maybe on the first face or the first and second faces of the protonexchange membrane. The catalyst layer components are formulated into anink as hereinbefore described and the ink deposited onto the first orfirst and second faces of the proton exchange membrane using techniquessuch as spraying, printing and doctor blade methods. Alternatively, thecatalyst layer is first formed on a transfer substrate (as hereinbeforedescribed) and transferred onto the proton exchange membrane using adecal transfer technique known to those skilled in the art.

The proton exchange membrane may be any membrane suitable for use in aPEMFC, for example the membrane may be based on a perfluorinatedsulphonic acid material such as Nafion® (DuPont), Aquivion® (SolvaySpecialty Polymers), Flemion® (Asahi Glass KK) and Aciplex® (AsahiKasei). Alternatively, the membrane may be based on a sulphonatedhydrocarbon membrane such as those available from FuMA-Tech GmbH as theFumapem® P, E or K series of products, JSR Corporation, ToyoboCorporation, and others. Alternatively, the membrane may be based onpolybenzimidazole doped with phosphoric acid which will operate in therange 120° C. to 180° C. Other components may be added to the membrane,for example to improve the durability, as will be known to those in theart.

The proton exchange membrane may be a composite membrane, wherein themembrane contains other materials that confer properties such asmechanical strength. For example, the membrane may contain a porousreinforcing material, such as an expanded PTFE material.

The proton exchange membrane may also comprise one or more componentswhich assist the chemical durability of the membrane, for example ahydrogen peroxide decomposition catalyst, a radical scavenger etc.

The invention also provides a MEA comprising a catalyst layer, a gasdiffusion electrode or a catalyst coated membrane of the invention andan electrochemical device, such as a fuel cell, comprising a MEA,catalyst layer, gas diffusion electrode or catalyst coated membrane ofthe invention.

Although the invention is described with reference to its use in aPEMFC, it can be understood that the catalyst layer of the inventionwill have application in other types of fuel cells where the highvoltage situations as described can occur. In addition the catalystlayer of the invention may find application in other electrochemicaldevices, such as at the oxygen evolution/reduction electrode of aregenerative fuel cell or the oxygen evolution electrode of anelectrolyser.

EXAMPLES

The invention will be described further with reference to the followingexamples, which is illustrative and not limiting of the invention.

Example 1

An anode catalyst ink was prepared by mixing an unsupported platinumblack catalyst (Johnson Matthey, HiSPEC 1000) with an aqueous dispersionof a perfluorosulphonic acid ionomer of equivalent weight 790 (SolvayPlastics, D79-25BS Dispersion) at a level of 16 wt % ionomer relative tothe weight of platinum. The aqueous solution had a total solids contentof 45%. The platinum black powder was added to the diluted ionomerdispersion at 65° C. with stirring. This dispersion was mixed using ahigh shear mixer to ensure the components were evenly dispersed beforeprocessing through a bead mill, to reduce the particle size of the Ptcatalyst. An oxygen evolution catalyst (Johnson Matthey, IrO₂/TaO₂prepared as described in WO2011/021034) was added to the diluted ink toachieve a ratio by weight of 1.0:1.26 Pt:IrO₂/TaO₂. The catalyst wasgently stirred into the ink to ensure even distribution. Afterprocessing the ink was diluted with neat propan-1-ol to 35% total solids(propanol concentration 30.9% total ink).

Synthetic graphite powder (Alfa Aesar, 46304), with an as-receivedaverage particle size of 7-11 micron, was dispersed in neat propan-1-ol.The graphite dispersion was mixed using a high shear mixer and added tothe Pt black/ionomer dispersion to give a weight ratio Pt to graphite of1:0.36. The amount of propan-1-ol added to the graphite was calculatedto give a final ink solids content of 16%. The final ink was passedthrough a high pressure homogeniser several times to further break downthe particles and to ensure a homogeneous composition. Characterisationof the ink via particle size analysis, measured in a 22%propan-1-ol/water solution revealed that the D50 of the particles (i.e.combined catalyst, graphite and ionomer particles) was 2.5 micron.

The anode catalyst layer was prepared by depositing the anode catalystink onto a PTFE decal-transfer substrate and drying to achieve acontinuous anode catalyst layer with a Pt metal loading of 0.08 mgPtcm⁻²as measured by XRF, and thickness of 2-3 μm. The same process was usedto produce a cathode catalyst layer decal at 0.4 mgPtcm⁻² from a cathodeink containing a supported 50% Pt/carbon catalyst and 80% ionomer withrespect to the weight of the carbon. The anode and cathode catalystlayer decals were positioned either side of a perfluorosulphonic acidmembrane and hot pressed to produce a CCM. Appropriate seals and GDLswere added to allow compatibility with the fuel cell single cellhardware and the resultant MEAs were evaluated for beginning of life(BOL) performance and cell reversal tolerance.

Comparative Example 1

A comparative MEA was prepared using the same methodology as describedfor Example 1. The anode electrocatalyst was a conventional carbon blacksupported platinum catalyst (Johnson Matthey, 60 wt % Pt/Ketjen EC300Jcarbon) and the ionomer dispersion was added to a level of 80 wt %ionomer relative to the weight of the carbon support. The anode layerformulation comprised the same ionomer type as Example 1, the same Ptloading, and the same OER catalyst, but does not comprise any graphite.The ratio of Pt to OER catalyst was 1.0:0.86 Pt:IrO₂/TaO₂. The othercomponents of Comparative Example 1 i.e. the GDL, seals, PFSA membraneand the cathode catalyst layer were identical to those used in Example1.

A summary of the Example 1 and Comparative Example 1 is given in Table1.

TABLE 1 Example 1 Comparative Example 1 Electrocatalyst Pt Black 60 wt %Pt/ketjen EC300J Oxygen evolution reaction IrO₂/TaO₂ IrO₂/TaO₂ catalystCarbonaceous material Graphite N/A Proton-conducting polymer PFSA EW 790PFSA EW 790 Platinum loading 0.08 mgPt/cm² 0.08 mgPt/cm² Catalyst layerthickness 2-3 μm 2-3 μm Pt-containing 1.0:1.26 1.0:0.86electrocatalyst:OER catalyst Pt:carbonaceous material   1:0.36 N/A

Example 1 and the Comparative Example 1 were both tested in the samesingle cell hardware under the same test conditions. Followingpositioning between the bi-polar plates of the cell, the MEA was placedunder compression, the cell was heated and reactant gases andhumidification supplied. The MEAs were conditioned and a beginning oflife MEA performance polarisation plot (voltage vs. current) wasmeasured under a range of operating conditions, including a test withH₂/Air, at 1.5/2.0 stoichiometry, 65° C., 50% relative humidity andambient pressure. The results for Example 1 and the Comparative Example1 (Comparative MEA) are shown in FIG. 1. The MEAs show a very similarvoltage and resistance response across the current density rangedemonstrating that the same BOL performance can be attained from theanode catalyst layer of the invention in comparison to the moreconventional carbon supported Pt/carbon catalyst based anode layer.

Following the BOL performance test the MEA performance stability duringa highly accelerated anode reversal test was evaluated. The MEA wasconditioned at 65° C., 50% RH and ambient pressure while supplied withH₂ and synthetic air (21% O₂/79% N₂) at 1.5/2.0 stoichiometry. The anodegas was then changed from H₂ to N₂. A load of 200mAcm⁻² was then drawnfrom the cell to simulate a reversal event caused by gas starvation tothe anode side. The cell voltage, resistance and CO₂ content of anodeexhaust was monitored for the 5 minute duration of the reversal event,after which time H₂ was re-supplied to the anode, and air to thecathode. The cell was reconditioned and a performance and resistancemeasurement performed under the same conditions as at BOL to determinethe impact of the reversal event. The procedure was repeated until a 50mV loss at 1.2 Acm⁻² on H₂/air was observed. This point was denoted asthe end-of-life (EOL) criteria. An anode diagnostic test was alsoperformed at BOL and after each reversal event in which the performancewas measured using a H₂/N₂ (75:25) anode gas composition to evaluate anychanges to anode mass transport losses throughout the test. FIG. 2compares the performance at 1.2 Acm⁻² after multiple reversal events forExample 1 and Comparative Example 1 (Comparative MEA) over the durationof the test. Superior retention of MEA performance is clearly observedfor Example 1 vs. the Comparative Example 1 under both H₂/air and in theH₂/N₂/air diagnostic test. Comparative Example 1 reached the EOLcriteria after only 21 reversal cycles, whilst Example 1 was clearlysuperior and was able to withstand 102 reversal cycles before reachingthe EOL point. This benefit is ascribed to the improved layerformulation of Example 1 minimising anode layer degradation via carboncorrosion (CO₂). This effect is seen in both the H₂/N₂ anode diagnostictest in FIG. 2 and the measured CO₂ in the anode exhaust during thereversal events. For Example 1 the CO₂ content of the anode exhaust wasalways measured to below 100 ppm, whilst for the Comparative MEA CO₂levels up to 470 ppm were measured in the anode exhaust.

1-16. (canceled)
 17. A catalyst layer comprising: (i) aplatinum-containing electrocatalyst; (ii) oxygen evolution reactionelectrocatalyst; (iii) one or more carbonaceous materials selected fromthe group consisting of graphite, nanofibres, nanotubes, nanographeneplatelets and low surface area, heat-treated carbon blacks; and (iv)proton-conducting polymer; wherein the platinum-containingelectrocatalyst is unsupported, supported on a non-carbonaceous supportor supported on the oxygen evolution reaction electrocatalyst.
 18. Acatalyst layer according to claim 17, wherein the platinum-containingelectrocatalyst is unsupported.
 19. A catalyst layer according to claim17, wherein the one or more carbonaceous materials have a surface areaof less than 100 m²/g.
 20. A catalyst layer according to claim 17,wherein the one or more carbonaceous materials includes graphite.
 21. Acatalyst layer according to claim 17, wherein the one or morecarbonaceous materials do not support the oxygen evolution catalyst. 22.A catalyst layer according to claim 17, wherein the catalyst layer is ≧1μm in thickness.
 23. A catalyst layer according to claim 17, wherein thecatalyst layer is ≦20 μm in thickness.
 24. A catalyst layer according toclaim 17, wherein the loading of platinum is ≦0.8 mg/cm².
 25. A catalystlayer according to claim 17, wherein the loading of platinum is ≧0.01mg/cm².
 26. A catalyst layer according to claim 17, wherein the ratio byweight of the platinum in the platinum-containing electrocatalyst tocarbonaceous material is from 5:95 to 95:5.
 27. An electrode comprisinga gas diffusion layer and a catalyst layer according to claim
 17. 28. Anelectrode according to claim 27, wherein the electrode is an anode. 29.A catalyst-coated membrane comprising a proton-conductive membrane and acatalyst layer according to claim
 17. 30. A membrane electrode assemblycomprising a catalyst layer according to claim
 17. 31. A catalyst layeraccording to claim 17, wherein the one or more carbonaceous materialsare graphite.
 32. A catalyst layer according to claim 17, wherein theone or more carbonaceous materials are selected from nanofibers andnanotubes.
 33. A catalyst layer according to claim 17, wherein the oneor more carbonaceous materials are nanographene platelets.
 34. Amembrane electrode assembly comprising an electrode according to claim27.
 35. A membrane electrode assembly comprising a catalyst-coatedmembrane according to claim
 29. 36. A membrane electrode assemblycomprising an electrode according to claim 28.